In the process of lithographic printing, also known as offset printing, printing plates are imaged with the data to be printed, processed chemically and mounted on the press. Almost all lithographic printing presses require the edge of the plate to be bent in order to attach it to the plate cylinder inside the press. Modern platemaking relies on Computer-to-Plate (CTP) platesetters, which expose the plate using high-powered lasers or UV light. After exposure, the plate has to be developed by running through a plate processor. Sometimes the plate is also run through an oven for increased durability. After processing, one or two edges of the plate are bent by a plate bender.
In order to improve the registration between the image and the bend (the bend locates the plate on the press), the plate is sometimes punched, either before or after imaging. Some CTP machines have built-in automatic punches in order to eliminate a manual step. The reason for the punching is historical: when plates were made from films, the holes were punched both in the film and in the plate and served to register the film to the plate. Many presses use the bend to locate the plate in the circumferential direction and one or more of the punched holes to locate the plate in the direction of the plate cylinder axis. Some presses do not rely on the punched hole at all, using just the bend and the plate edge to register.
The punching of the plate can be done before imaging, while the plate is in the CTP platesetter, after imaging of the plate but before its processing, or after processing of the plate. When the punching is done as part of the imaging process in the CTP platesetter, it is fully automated. The reason why the bending could not be automated in the same way, is simple: the plate has to be flat in order to be processed, as the processing relies on the uniform nature of a flat plate to expose each part equally to the action of the processing chemicals. This is also the reason why, whenever some bending inside the CTP platesetter was required in the prior art (for example, to curve the plate for a better fit to the drum), any residual bend had to be straightened out before the plate could be fed to the plate processor. The art of platemaking, including CTP platesetters, has been known for at least 20 years and needs no further explanation here. CTP platesetter machines are available from vendors such as Creo (Canada). Automatic punching and bending systems are available from vendors such as Nela-Ternes (USA).
Recently a new type of plate that does not require processing became available for CTP use. Such plates are known as processless or “chemical free” plates. Examples of such plates include: Saphira (sold by Heidelberg of Germany); Applause and Anthem (sold by Presstek of N.H., USA) and Navajo (sold by Kodak Poychrome Graphics, USA). It is an objective of the present invention to provide an apparatus and method by which the manual step of plate bending is eliminated, and the properties of processless plates are employed to bend the plate automatically in a CTP platesetter machine. The full advantages of combining the proprties of processless plates with the step of bending inside the CTP platesetter will become apparent from the following disclosure.